1. Field of the Invention
The present invention relates to aqueous regenerative SO.sub.2 scrubbing systems which use a sulfite-forming SO.sub.2 absorbent.
2. Description of the Prior Art
The aqueous regenerative SO.sub.2 scrubbing system of which the present invention is an improvement is fully described in the above-related applications. It comprises a scrubbing circuit and a regenerative section. In the scrubbing circuit, there is a scrubber through which an aqueous scrubbing solution continuously circulates in contact with the SO.sub.2 -containing gas. A substantially constant high concentration, i.e. at least ten percent (10%) by weight of either sodium or potassium thiosulfate is maintained in the aqueous scrubbing solution throughout its traverse around the scrubbing circuit. The effective SO.sub.2 absorbent is not thiosulfate but rather is either sodium or potassium carbonate dissolved in the aqueous scrubbing solution. The term "carbonate" as used herein includes both carbonate (CO.sub.3.sup.= ) and bicarbonate (HCO.sub.3.sup.- ). The carbonate is converted by reaction with SO.sub.2 to sulfite in the scrubber under sulfite-forming conditions. Similarly, the term "sulfite" as used herein includes both sulfite (SO.sub.3.sup.= ) and bisulfite (HSO.sub.3.sup.- ). The letter "M" as used hereinafter means either sodium or potassium.
A second very rapid reaction occurs in the scrubbing circuit but external to the scrubber. That reaction is the conversion of sulfite by MHS to thiosulfate (M.sub.2 S.sub.2 O.sub.3) in a sulfite conversion zone, whereby the sulfite concentration in the scrubbing solution is reduced to very low levels.
The required carbonate and the required MHS are produced in the regenerative section by reacting the M.sub.2 S.sub.2 O.sub.3 which is withdrawn from the scrubbing circuit in a slipstream with a reducing gas containing CO at an elevated temperature and pressure. The regeneration reaction may be expressed as: EQU 3 M.sub.2 S.sub.2 O.sub.3 + 12 CO + 5 H.sub.2 O .fwdarw. 2 M.sub.2 CO.sub.3 + 2 MHS + 4 H.sub.2 S + 10 CO.sub.2 1.
however, in reality, there appear to be several side reactions in which products may react with the reactants and with each other. Accordingly, careful control of the conditions under which reaction (1) is conducted is essential to form the required quantities of carbonate and MHS for return together in the regenerated aqueous solution to the scrubbing circuit to repeat the cycle.
The maintenance of the proper ratio of MHS to carbonate in the regenerated solution which is returned to the sulfite conversion zone in the scrubbing circuit is essential to satisfactory operation of the scrubbing and regenerative cycle. That ratio is maintained provided the following ratio, R, is maintained at or below 1 in the regenerated solution, preferably between 0.75 and 0.98. ##EQU1## where: (S.sup.o ) = gram atoms sulfur with valence number zero/100 grams solution.
(S.sup..sup.-2) = gram atoms sulfur with valence equal to -2/100 grams solution. PA1 .SIGMA.M = gram atoms of M/100 grams solution present in said aqueous effluent as MHS, M.sub.2 S, M.sub.2 S.sub.x, M.sub.2 CO.sub.3, MHCO.sub.3, and MOH (where M is Na or K).
When R is greater than 1, there will be excess MHS and associated sulfides in the regenerated solution. Such excess will reach the scrubbing zone of the scrubbing circuit where it is converted to H.sub.2 S by hydrolysis and/or CO.sub.2 stripping. When R is less than 0.75, there is insufficient MHS and associated sulfides to maintain the sulfite concentration of the recirculating solution at the desired predetermined low level.
Prior art patents which describe processes similar in some respects to the above-described process include the following: U.S. Patent Nos. 1,937,196, H. A. Gollmar, Nov. 28, 1933; 2,729,543, J. L. Keller, Jan. 3, 1956; 3,431,070, J. L. Keller, Mar. 4, 1969; 3,574,097, P. Urban, Apr. 6, 1971; 3,635,820, P. Urban, Jan. 18, 1972; 3,644,087, P. Urban, Feb. 22, 1972; 3,714,338, P. Urban, Jan. 30, 1973; 3,859,416, P. Urban, Jan. 7, 1975.
3. The Problem
The ratio, R, may be established and maintained at or below 1 by conducting the reduction of thiosulfate in a single reduction zone. This zone, as will be later described, operates at a temperature and pressure which are much higher than those maintained in the scrubbing circuit. It has been discovered that the value of R undergoes a significant decrease in the course of the return of the effluent aqueous product from a single reduction zone to the scrubbing circuit; and may even be of such magnitude as to cause the value of R to fall below the desired range of 0.75 to 0.98.
Accordingly, the primary object of this invention is to provide for the assured return to the sulfite conversion zone of the above-described scrubbing circuit of a regenerated aqueous solution having not only the desired value of the ratio, R, but also a value of R which is stable.